Design of an efficient tannin extract effluent treatment system

A study was undertaken on the combined wastewater effluent from a Tannin ex tract plant (90%) and a Mushroom Production Unit (10%). The effluent had BO D5 of 2350 mg/l, COD of 4600 mg/l, and high levels of total suspended solid (0.43-4.20 mg/l), phosphorus (2078 mg/l), free ammonia (2982 mg/l) and col our index of 50 to 150 degrees H. These values are effluent Standards of El doret Town in Kenya. To address these problems, three potential treatment p rocesses were considered: VITOX system using pure oxygen, lagoon evaporatio n system and Step-alum coagulation/flocculation process using wood ash leac hate for pH adjustment. The latter was chosen because of its efficiency for not only BOD5, COD, phosphorus and free ammonia reduction but also for the complete colour removal when alum was added in two stages as laboratory Ja r-tests indicated. A plant was subsequently designed that incorporated a tw o-step alum coagulation/flocculation process in two upflow tanks with wood- ash leachate for pH adjustment during and after flocculation. (C) 1999 Publ ished by Elsevier Science Ltd on behalf of the IAWQ. All rights reserved.